The present embodiments relate to radiometry using an uncooled Microbolometer detector and more particularly but not exclusively to obtaining more accurate results from an uncooled Microbolometer so as to extend its useful range of applications into areas previously only feasible with the more power consuming and more complex vacuum packages. Infra-red (IR) detectors detect the IR radiation emitted from an object, and are used for non-contact measurement of temperatures in many industrial and medical applications. These applications include stress testing electronic components, measuring human temperature, surveillance systems including long range and nighttime surveillance systems, fire detection, and portable night vision equipment.
IR detectors generally operate by detecting the differences in the thermal radiance of various objects in a scene. The difference is converted into an electrical signal which is then processed and analyzed and/or displayed. Imaging radiometers, such as forward-looking IR (FLIR) cameras, utilize an array of IR sensors to provide a two-dimensional thermal image. The more simple detectors are typically used to provide an image, since a simple differential over the field of view provides an image which can be meaningful to the eye. However, generally more sophisticated equipment is required if the intention is to measure a temperature from the received radiation.
In many cases the sensor array is a microbolometer array. Microbolometers are IR radiation detectors that are fabricated on a substrate material using integrated circuit fabrication techniques. Microbolometer detector arrays may be used to sense the incident radiation. Each microbolometer detector of an array absorbs incident radiation which leads to a corresponding change in its resistance due to its change in temperature. With each microbolometer functioning as a pixel, a two-dimensional image or picture representation of the incident infrared radiation may be generated using a suitable array of the microbolometers.
FLIR cameras have non-uniform responses to uniform incident infrared radiation. This is due to:                a. small variations in the detectors' electrical and thermal properties as a result of the manufacturing process,        b. variation of the electromagnetic wave intensity as a function of emitting/absorbing angle,        c. optics vignetting,        d. optics change in temperature, etc These non-uniformities in the microbolometer response characteristics are generally corrected to produce an electrical signal with adequate signal-to-noise ratio for image processing and display.        
As is well known in the art, offset and gain information associated with each of the detectors is obtained by a calibration technique known as non-uniformity correction (NUC), in which the microbolometer array is subjected to several scenes of uniform radiation at different levels. During regular use a controlled shutter is closed to obscure the microbolometer array's field of view (FOV), so that all of the sensors view a uniform temperature scene. The response of each of the detectors is used to derive a corresponding offset value. The shutter is then opened, and normal imaging operation is resumed. The derived offsets are used to correct the response from each pixel in the array. Furthermore, in order to compensate for the optics and new temperature distributions within the internal camera parts the detector itself is stabilized in temperature so that its response is relatively stable. While the shutter blocks the detector's FOV, an additional correction of the microbolometer array response, known as bad pixel replacement (BPR) updating, may be performed. The signal from each detector pixel is checked to determine whether the detector pixel is functioning properly. If the detector pixel is found to be inoperative, or its signal properties drastically differ from the average properties, then the value of the corresponding pixel is determined by other means, such as taking an average of the surrounding pixels. An alternative technique is to perform sorting instead of averaging.
There are several main obstacles related to building an imaging radiometer, especially when the instrument is based on an uncooled microbolometer detector array. A simple uncooled microbolometer array detector does not contain any radiation shield. That is to say in the more sophisticated detectors there are radiation shields which protect the detector from IR radiation from internal camera parts. The simple detectors do not contain such a shield, making the package smaller and the optics simpler, but meaning that the simple detector exchanges energy with the internal camera parts and its vacuum package through a solid angle of 2π radians.
Indeed it is noted that the detector always exchanges energy through a solid angle of 2π radians. However, conventional more sophisticated radiometric detectors block the detector field of view using the above mentioned radiation shields. Furthermore the radiation shields are kept at a constant temperature, which can therefore be compensated for relatively easily. Returning to the simple detectors and in fact most of the energy exchanged is between the detector and the internal camera. After all, the internal camera parts are much nearer than the objects being imaged. Only a very small fraction (usually about 10%) of the energy exchanged by the detector comes from the scene it is intended to be imaging. A very small change in temperature of the internal camera parts therefore may produce a large change of the detector output signal. The average value and the very low frequency components of the video signal obtained from the microbolometer detector after the processes of NUC and BPR are greatly influenced by the temperature of the detector's vacuum package and internal camera parts.
One set of solutions is the cryogenic cooled detectors arrays. Cooled detectors have the disadvantages of greater weight and complexity, as well as additional power consumption for cooling, shorter lifetimes and greater cost. Advantages of cooled detectors are that they can work at shorter wavelengths, say the 3 to 5 micron band and thus produce images having greater resolution. FLIRs based on the cooled devices have good sensitivity even for relatively high f numbers (f#) and have a very short time constant.
On the other hand, the uncooled thermal microbolometer array has other advantages. For example, uncooled devices have smaller physical size, lower weight and low power consumption.
The uncooled devices give video output immediately after power on, have a long MTBF (mean time between failures) relative to the cooled devices, and are generally cheaper.
Overall the cooled devices are used for long range applications and those in which the greater expense can be justified and the uncooled devices are used for medium and short range applications and those in which budgets are limited.
A second solution to the above described problem is proposed in U.S. Pat. No. 6,476,392 by Kaufman et al., which presents a temperature dependent focal plane array that operates without a temperature stabilization cooler and/or heater. Gain, offset, and/or bias correction tables are provided in a flash memory, in memory pages indexed by the measured temperature of the focal plane array. The gain, offset, and/or bias for each pixel are determined at each small temperature increment over the entire temperature operating range, for example by placing the array in a controlled oven and examining the array's response to a known temperature. The bias, gain, and offset data within the database are later read out, converted to analog form, and used by analog circuits to correct the focal plane array response. The data used for signal correction is determined only once, and then not under normal operating conditions. Kaufman et al. do not account for the variation in detector response over time, or due to other factors. Furthermore, Kaufman's patent deals with the detector response for the case that the detector does not have a fixed temperature stabilized working point. In such a case, temperature measuring requires a complete new data set for each small temperature increment. Such an approach is impractical.
An additional problem for imaging IR radiometers is that microbolometer detector arrays generally have a finite spatial response. Two black bodies that have the same temperature, but differ in size, or the same object at the same temperature but at different distances from the camera, produce different video signals at the detector output. A direct translation of the differential video signal into temperature is relatively accurate only for large objects. More specifically, the translation of the video signal into temperature is subject to an error introduced by the limited modulation transfer function (MTF) response for high spatial frequencies.
There are currently available a large number of uncooled microbolometer FLIR (forward looking infra-red) cameras, covering a large number of applications. These cameras contain regular uncooled microbolometer sensors that do not contain any radiometric shield. Typically, these cameras come with a standard lens of 35 mm or 50 mm focal length or other focal length optics. A problem is to find a way to upgrade these existing cameras in order to overcome the above-described drawbacks of the uncooled detector to allow them to give accurate temperature measurements under the restriction of very minor hardware modifications.